U.S. Pat. No. 3 843 736 discloses a process for isolating vinyl chloride from the reaction mixture obtained in the incomplete thermal cracking of 1,2-dichloroethane at a superatmospheric pressure of preferably from 8 to 40 atm. absolute and temperatures between about 450.degree. and 650.degree. C. in the absence of catalysts, with the gas mixture leaving the cracking zone, comprising vinyl chloride, hydrogen chloride, unreacted 1,2-dichloroethane and byproducts, being conveyed in partially condensed form into a first distillation stage to distill off the hydrogen chloride and subsequently being conveyed into a second distillation stage to distill off the vinyl chloride, the bottom product of the second distillation stage then being worked up in a known manner for the purpose of recycling the unreacted 1,2-dichloroethane, and with the vinyl chloride drawn off at the top of the second distillation stage and liquefied being partially pumped back into about the middle of the first distillation stage, in which process the vinyl chloride drawn off and liquefied as product, which still contains small amounts of hydrogen chloride, is fed to the top of a degassing zone from which a part of the vinyl chloride is evaporated by means of a circulation evaporator together with the entire entrained hydrogen chloride, the evaporated vinyl chloride is condensed and pumped back into about the middle of the first distillation stage provided for distilling off the hydrogen chloride, while very pure vinyl chloride is taken off from the bottom of the degassing zone.
This patent also mentions that the small amounts of hydrogen chloride which are removed in the degassing zone were formerly separated off by means of towers filled with sodium hydroxide.
However, the known process is disadvantageous if the vinyl chloride fed to the degassing zone still contains small amounts of water. These are unavoidable in practice; especially when running up the plant and during interruptions in operation, traces of water are unavoidably introduced. In practice, therefore, the degassing zone was then operated in such a way that these amounts of water were separated off via the bottom of the degassing column, together with hydrogen chloride, for which purpose the towers filled with sodium hydroxide were again required. If this separation of the water were not carried out via the bottom of the degassing column, the water would be circulated together with the recycled vinyl chloride and lead to corrosion in the affected parts of the plant. However, such corrosion is unacceptable not only for economic reasons, but especially for safety reasons.